Zapped

by Bob Berman

  In the midst of this commotion, several brilliant physicists could scarcely sleep: the recently solved mysteries actually seemed minuscule next to the ones that remained. But many of these mysteries were hovering on the cusp of elucidation. Two of these physicists were a married couple who in a few short years would be known around the world—Pierre and Marie Curie. Another was New Zealand–born Ernest Rutherford, who ultimately revealed the nature of the atom. The fourth member of our 1890s science quartet is Antoine Henri Becquerel, born in Paris in 1852 to a distinguished family of scholars and scientists. Each opened the first doors to our understanding of nature’s unseen entities.

  Becquerel was a French professor of applied physics; his passion in the 1890s was phosphorescence—the odd emission of colored light after a substance is exposed to light of another color. (Imagine shining red light on a particular odd-looking rock only to see it emit green light after nightfall.) When Röntgen discovered X-rays, Becquerel assumed that phosphorescent materials such as uranium salts might glow simply by emitting some X-ray-like radiation after being stimulated by bright light.

  In February of 1896, that watershed year for invisible light, Becquerel exposed uranium to sunlight and placed it on photographic plates he had first wrapped in thick black paper. Sure enough, when he developed the film, it revealed an image of the uranium crystals. He concluded, “The phosphorescent substance emits radiation which penetrates paper opaque to light.” He assumed that the sun’s energy, absorbed by the uranium, was then released as X-rays or something similar to them.

  Becquerel planned to repeat the experiment on February 26 and 27, but on those days Paris was overcast. With no sunshine to which he could expose his uranium, he put the wrapped-up photography plates in a drawer, along with his uranium. Two days later, on a hunch, he developed the plates, expecting to see only faint images because the uranium had not been stimulated by anything other than dim house light. Instead the images of the uranium crystals were vivid.

  There was only one explanation: uranium was independently emitting its own invisible rays. It didn’t require any prestimulation by an external energy source. Meaning that some materials spontaneously emit energy on their own. They don’t need to be first excited by light or heat. Becquerel initially assumed that this emission was X-rays or some similar unknown invisible light. But when he performed a critical magnetic test to settle the matter, he found that the emission’s path was bent by a magnetic field. Neither X-rays nor any other light deflects when passing near a magnet, which meant that the uranium couldn’t be emitting any sort of unseen ray. Instead it must be giving off hordes of tiny charged particles, like a swarm of bullets. Becquerel had discovered radioactivity, though he failed to give it a name. For this he was awarded the 1903 Nobel Prize in Physics, which he shared with the Curies.

  Hearing of Becquerel’s experience with uranium, Ernest Rutherford started to explore its radioactivity and soon changed our knowledge of nature at its most fundamental level. Born in 1871, Rutherford was raised on the mostly rural South Island of New Zealand, the fourth of twelve children. His father, a flaxseed farmer, struggled financially, while his mother, Martha, was a poorly paid schoolteacher. As a child, Rutherford was painfully aware of his family’s financial struggles. They even supplemented their income by collecting birds’ nests so the family could use the eggs.

  Determined to succeed in the world, Rutherford earned a degree at what was then known as the University of New Zealand and was awarded a scholarship to Cambridge. There he became Joseph John Thomson’s first graduate student (before Thomson discovered the electron). And it was there that he turned to exploring radioactivity.

  Using a more advanced measuring technique than the mere fogging of photographic film—namely, the degree to which radioactivity ionizes the surrounding air so that electric current can penetrate it more easily—he found that uranium as well as thorium emissions have a double component. One component, which he soon named alpha radiation, was absorbed and blocked during his experiments by just a few thousandths of an inch of metal foil, beyond which it was undetectable. But the other component, which he named beta, easily passed through one hundred times as much foil before it vanished.

  Moreover, when he subjected each component to magnetic fields, he found that while both their paths were deflected, the beta rays shifted dramatically while the alpha rays barely budged at all. This told him that the beta rays must actually be lightweight particles with an electric charge whereas the alpha particles must be heavy and neutral, or chargeless.

  Rutherford later accepted a teaching post at McGill University, in Montreal, where he continued his research. His terms alpha rays and beta rays became globally accepted in 1899 as a way to describe the two distinct types of radiation.

  In 1902, working with uranium, thorium, and radium—a brand-new substance discovered by Marie and Pierre Curie that emitted far more intense radiation than either uranium or thorium—Rutherford and his assistant, Frederick Soddy, formed a theory of atomic disintegration to account for all the oddities their experiments were revealing. Until that year, atoms were assumed to be the stable, eternal, unalterable basis of all matter. But Rutherford and Soddy showed that radioactivity is actually the disintegration of atoms into other types of atoms, along with their components. By definition, in other words, radioactivity is a process of disintegration. Here was a phenomenon the alchemists had tried to reproduce for centuries: one element being changed to another.

  In 1903, Rutherford studied a powerful new type of radiation emitted by radium and found that it had amazing penetrating power. This radiation, which had been discovered by the French chemist Paul Villard, was very different from his alpha and beta rays. Rutherford naturally named this third type of radiation the gamma ray. He found it could easily penetrate metal foil; it even survived after passing through several inches of lead. Gamma rays did not deflect—not in a magnetic field, not in an electric field. They always went straight ahead, which meant they must be a form of invisible light rather than particles.

  Rutherford did more than merely find and identify various kinds of radiation. Along with the Curies, he described their natures. When the dust cleared, radiation was finally explained. It turns out that heavy radioactive elements have such massive nuclei that pieces of them spontaneously break off. Thus an alpha ray was initially (and correctly) called an alpha particle—a massive chunk consisting of two protons and two neutrons, which commonly exists in nature as the nucleus of helium, the second-most-common element in the cosmos.

  Beta particles turned out to be electrons. Simple lightweight electrons, nothing more. They have a negative charge of one.

  Rutherford found that a gamma particle is the only type of radiation that is in fact not a particle but a ray of light, with no charge and no weight. History, it seems, saved the strongest for last: gamma rays are not only the final variety of invisible light to be discovered but also the most energetic, with the power to damage any atom or molecule. We’ll explore these mighty rays in the next chapter.

  In 1909, Rutherford and two assistants, including Hans Geiger of Geiger counter fame, performed the now-famous gold foil experiment. Because gold is so ductile, it can be pressed or hammered into a foil thinner than that of any other metal. Rutherford fired a beam of alpha particles at a layer of gold leaf only a few atoms thick.

  At the time, every atom was thought to be like a lump of pudding, a hypothesis proposed by Joseph John Thomson. The atom’s negative charge was thought to be scattered like little raisins throughout a positive sphere. But if this pudding model were correct, the atom’s positive components would be spread out rather than concentrated, pointlike, in the center. Moreover, alpha particles hurled at foil made up of heavy gold atoms would only be deflected by small angles as they passed through, because the heavy alpha particle would never encounter a massive enough obstruction to seriously alter its high-speed trajectory.

  But that’s not what happened. The amazing result was
that one out of every eight thousand hurtling alpha particles was deflected by a very large angle of more than ninety degrees, while the rest passed straight through with little or no deflection. From this Rutherford concluded that the majority of an atom’s mass must be concentrated in a tiny, positively charged region, with electrons surrounding it like planets orbiting the sun.

  Many years later, reflecting on his experiment, Rutherford said: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.”

  Using mathematical analysis, Rutherford proposed, in 1911, a model for the atom that is still accepted today. He concluded that all the positive charge and essentially all the mass of the atom is concentrated in an infinitesimally small fraction of its total volume. This core is ten thousand times smaller than the atom itself. He called this the nucleus, from the Latin for “little nut.” The electrons orbit far away from the nucleus and from each other; they’re in a mostly vacant realm. Thus the vast majority of the volume of an atom is empty space.

  And though this makes us momentarily jump ahead in our story’s chronology, it must be mentioned that in 1917 Rutherford proved that multiple copies of a hydrogen nucleus are present in every other atom’s nucleus—and he proposed in 1920 that this positively charged component in every atom be called the proton. That same year, he said that an atom’s nucleus must also contain a separate, different kind of massive particle, one that has no charge at all—essentially a proton somehow fused together with an electron. He suggested that these theoretical subatomic particles, each with a neutral charge and a mass equal to a proton’s plus an electron’s, be called neutrons. The name stuck, even if actual neutrons were not discovered for another dozen years.

  Rutherford was right about all of it. Today we know that yes, every atom’s nucleus consists of protons and neutrons. And yes, this nucleus is tiny, yet it contains virtually all the atom’s weight. Each proton is 1,836 times more massive than each electron orbiting it. Yet this heft is confined in an unimaginably minuscule volume.

  To be so small yet have such mass means that protons and neutrons are astonishingly dense. To equal their density, you’d have to crush a cruise ship down until it was the size of the point in a ballpoint pen. Imagine a sphere smaller than a mustard seed, weighing what a cruise ship does and containing every ton of its steel. Seems impossible, right? And yet that exact density exists in each of the one hundred billion billion billion protons and neutrons in each of our bodies.

  It is true that when Becquerel and Rutherford were making their groundbreaking discoveries about radiation and the atom, the discovery of important underlying mechanisms of physics still lay in the future. Einstein’s relativity theories of 1905 and 1915—and the astounding quantum theory of Niels Bohr and Max Planck and its refinements by Paul Dirac, Erwin Schrödinger, and others—had not yet been developed. Nonetheless it is obvious that thanks to these physicists and the Curies, our knowledge of nature made an enormous leap in the years surrounding the turn of the century.

  Marie Curie was born in Poland in 1867 as Maria Sklodowska and was forever bound to that country in her heart, insisting that both her daughters become fluent in Polish long after she’d immigrated to France. An autodidact, she spent her adolescence reading books and becoming obsessed with science. In 1889 she started working as a governess in Warsaw, supporting herself while studying at Flying University and finally pursuing scientific training in a chemical laboratory in downtown Warsaw.

  She left Poland for Paris at the end of 1891 to live at first with her sister and brother-in-law before finding a tiny apartment in the Latin Quarter and studying chemistry, physics, and mathematics at the University of Paris. Life was difficult; she was almost always destitute and occasionally fainted from hunger.

  Studying during the day and tutoring in the evenings, she nonetheless earned her degree in physics in 1893 and was hired by an industrial laboratory. After earning a second degree in 1894, she met the love of her life, Pierre Curie, a young instructor at the Paris Municipal School of Industrial Physics and Chemistry. They bonded over their shared love of science and quickly discovered mutual interests, such as taking long bicycle trips. They were married in the summer of 1895.

  Near the end of that year, Wilhelm Röntgen discovered X-rays, and the Curies were swept up in the uproar that followed. Much about this form of invisible light was still mysterious. Discoveries were following one another like boulders in an avalanche. A few months later, early in 1896, Antoine Henri Becquerel showed that uranium emitted rays whose penetrating power closely matched X-rays and that this emission seemed to arise spontaneously from the uranium itself. While the world reeled from these findings, Marie decided to research these new rays herself.

  Her first breakthrough came along when she discovered a far more precise way of quantifying uranium’s emissions (which Marie, in a published paper, termed radiation, a label that permanently stuck) than the simple method of fogging photographic film. More than a decade earlier, Pierre and his brother had developed a sensitive instrument for measuring the strength of an electric charge. Using this device, called an electrometer, she measured the intensity with which uranium rays caused the air around them to conduct electricity. It was a vast improvement over the previous method, and it actually underlies the principle behind the Geiger counter, an invention that would come along later.

  Marie’s very first results showed that the amount of detected radiation varied with the weight of the uranium, forcing her to conclude that the emissions came from the element’s atom rather than from some interaction with external light or other substances. This was the first step in her groundbreaking discovery that atoms are not permanent and indivisible but can break down and mutate.

  In July of 1898, Marie and Pierre published a paper under joint authorship announcing the existence of an element they named polonium, in honor of Marie’s native land. Six months later, on the day after Christmas in 1898, the Curies announced their discovery of a second element, which was an amazing one million times more radioactive than uranium. They named it radium, from the Latin word for “ray.”

  Alas, radium was present in such minuscule amounts in radioactive ores that it proved nearly impossible to isolate in pure form—normally the first and most essential step in nailing down the properties of any new substance. (The goal is to eliminate impurities that can confound future tests.) Finally, in 1902, using one ton of uranium’s main ore, pitchblende, the Curies managed to produce one-tenth of a gram of radium chloride. It then took until 1910 for Marie to isolate pure radium in its metallic state. But it was an important quest for her. She’d made the discovery that when exposed to radium, tumors shrink, and she soon pioneered its medical use, regarding it as a miracle substance. For her whole life, she alluded to it as “my beloved radium.”

  Pierre died suddenly in Paris on a rainy, windy night in 1906, run over by a horse-drawn carriage whose wheels fractured his skull. Marie, her fame only growing, lived for another quarter century. She succumbed to aplastic anemia in 1934 during a visit to Poland, done in by the very substance—radium—whose name she’d coined. She had routinely carried unshielded vials of radium in her lab coats as they silently emitted the most powerfully penetrating invisible light ever discovered—gamma rays. To the end, she never admitted that radium carried any peril. However, her notebooks, more than a century later, still remain too radioactive to handle. To this day, special suits must be worn to peruse them.

  For her work, Marie won the Nobel Prize in 1903, sharing it with Antoine Henri Becquerel and her husband, Pierre. She was the first woman to win the prize and the first person to win a second Nobel, in 1911.

  CHAPTER 17

  Gamma Rays: The Impossible Light

  To harm us, light has to penetrate skin and then damage cells. Visible light can’t do this, and neither can radio waves, microwaves, or infrared ra
diation. But the highest-energy forms of light, X-rays and gamma rays, zoom into our bodies as if our skin were made of mist.

  By the final few years of the nineteenth century, scientists were getting better at detecting radiation. At first the fogging of photographic plates wrapped in black paper was the only available way to see these emanations. Using this technique was a quick and easy way to determine which substances did or did not create radiation. For example, no fogging of film ever followed exposure to Herschel’s “calorific rays” or Hertz’s radio waves. Even Ritter’s “chemical rays” (ultraviolet rays), despite their higher frequency and greater energy, didn’t leave markings on sealed photography plates. But Röntgen’s rays did. They alone—along with the soon-to-be-discovered gamma rays—had sufficient penetrating power.

  By the time the Curies discovered radium, in 1898, more refined methods of detecting unseen emissions were available. As they pass through air, charged particles and high-energy light knock electrons from the neutral atoms of gaseous oxygen and nitrogen, leaving them with an electrical charge. This charge, in turn, can be measured by its ability to conduct electricity through the gas.

  We’ve seen that in 1899, Rutherford named Becquerel’s easily blocked radioactive emanations alpha particles and labeled those with greater penetrating power beta particles. In 1900, Paul Villard, a forty-year-old Parisian schoolteacher who maintained a small laboratory, started to study the emissions of the element radium. He put a piece of radium in a lead box that had a small opening from which its “rays” would stream. He saw and recognized the previously described radium rays and realized that something new was also streaming out, something more powerful and penetrating than anything previously observed. A modest man, Villard did not give this superpower emission a new label but merely described its properties. Three years later, in 1903, Ernest Rutherford named Villard’s discovery gamma rays to keep his Greek-letter nomenclature system intact.